The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present invention is related in general to equipment for servicing subterranean wells. Particularly, the invention relates to a cementing plug that is equipped to activate autofill float equipment.
During a cementing operation, the primary purpose of float equipment is to allow operators to pump cement slurries into the well that are heavier than the drilling fluid. After cement-slurry placement, check valves prevent the slurry from flowing from the annulus back inside the casing or liner string—a phenomenon often called “U-tubing.” Such float equipment may be float shoes or float collars
Autofill float equipment contains check valves similar to those employed in conventional float shoes and collars. However, the check valves are modified to remain in the open position to allow filling or even reverse circulating. The tubular string fills continuously as it is run downhole, saving rig time and reducing the pressure surges associated with conventional float equipment.
Autofill equipment must be activated, or converted, to begin functioning as a one-direction check or float valve. Conversion is generally performed after the tubular string is in place; however, it can also occur while running the tubular string to prevent overflow or to control the well. A thorough summary of conventional and autofill float equipment is presented in the following reference: Leugemors E, Metson J, Pessin J-L, Colvard R L, Krauss C D and Plante M: “Cementing Equipment and Casing Hardware,” in Nelson E B and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006): 343-434.
A typical technique for activating autofill float equipment is depicted in FIG. 1. The technique is shown in five steps, shown as A through E. Step A shows an autofill float collar 1 mounted at the bottom of a tubular string 2. As the tubular string is lowered into the subterranean wellbore, the flow direction of wellbore-service fluid (e.g., drilling fluid) through the autofill float collar and tubular string is upward 3. In Step B, the wellbore-fluid circulation direction is reversed 4 so that fluid travels down the tubular string, through the float collar and up the annulus between the tubular string and the wellbore wall. A ball 5 has been launched inside the tubular string, and is traveling down toward the autofill float collar. Step C shows that the ball has entered the autofill float collar and become seated in an orifice tube 6. In Step D, continued pumping of wellbore service fluid increases the pressure above the ball, causing shear pins 7 to rupture and release the orifice tube. Ejection of the orifice tube exposes one-way flapper valves 8 and 9, allowing activation of the float collar. The flapper valves close, thereafter allowing downward fluid flow but preventing fluid flow in the upward direction. Step E depicts the autofill float collar after activation is complete.
The prior art method described in the preceding paragraph is generally reliable when applied in near vertical wells, usually up to about 30° deviation. At higher deviations, up to and including horizontal wells, the rate at which the ball travels to the float collar may not be sufficiently high, or the ball may become stuck and never reach the float collar. Failure to activate the autofill collar would allow annular fluids to reenter the tubular string.
This problem has previously been mitigated by preinstalling the activation ball in a cage mechanism located above the autofill valve, where it remains until downward circulation begins. Circulation flow forces the ball into the autofill float collar, build up backpressure and activate the valve. The limiting factors are that there is less control of valve activation, and the ball may restrict fluid flow and the solids carried therein. Another option is to locate the ball in a mechanism further uphole; however, there is still no direct control of when the autofill-valve activation takes place.
It therefore remains desirable to provide improvements in the control and reliability of equipment for activating autofill equipment.